1. Field of the Invention
The present invention relates to a dielectric ceramic which is advantageously used in a laminated ceramic electronic element such as a laminated ceramic capacitor having an internal conductor formed of a base metal such as nickel or nickel alloy, and to a method for producing the dielectric ceramic. The present invention also relates to a laminated ceramic electronic element which is formed of the dielectric ceramic and to a method for producing the same.
2. Description of the Related Art
Miniaturization and cost reduction of laminated ceramic electronic elements is in progress. For example, the ceramic layer has been thinned and a base metal has been employed as an internal conductor in such a ceramic electronic element. In the case of a laminated ceramic capacitor, which is one type of laminated ceramic electronic element, the dielectric ceramic layer has been formed as thin as about 3 xcexcm and a base metal such as Cu or Ni has been employed as a material for producing an internal conductor, i.e., an internal electrode.
However, when the ceramic layer becomes thin, the strength of an electric field applied to the layer increases and causes a problem in the ceramic layer dielectric exhibits a great change in dielectric constant induced by the electric field. Decrease of the size of ceramic grains in the thickness direction of the ceramic layer also causes a problem in reliability.
In order to cope with such situations, Japanese Patent Application Laid-Open (kokai) Nos. 9-241074, 9-241075, etc. have proposed ceramic materials which enable enhanced reliability by increasing the size of ceramic grains in the thickness direction of the dielectric ceramic layer. Thus, controlling the grain size of ceramic grains allows a reduction in change of dielectric constant induced by an electric field or temperature.
However, when a barium titanate material exhibiting a strong dielectric property is used as a material for a dielectric ceramic layer having a thickness of about 1 xcexcm or less in the above-described conventional art, the effect of electric field intensity on the dielectric ceramic layer increases to thereby lower the dielectric constant considerably. When a laminated ceramic electronic element is constructed thereof, the rated voltage must be lowered. Therefore, realization of a thin layer having a thickness as thin as 1 xcexcm or less is difficult or impossible so long as the above-described conventional art is employed to solve the problem.
In view of the foregoing, the present invention is directed to a dielectric ceramic which is advantageously used in a laminated ceramic electronic element including a thin ceramic layer having a thickness as thin as about 1 xcexcm or less and to a method for producing the dielectric ceramic. The present invention is also directed to a laminated ceramic electronic element which is formed of the dielectric ceramic and to a method for producing the same.
In one aspect of the present invention, there is provided a dielectric ceramic which is obtained by firing a barium titanate powder having a perovskite structure in which the c-axis/a-axis ratio in the perovskite structure is about 1.000 or more and less than about 1.003 and the amount of OH groups in the crystal lattice is about 2.0 wt. % or less.
In another aspect of the present invention, there is provided a method for producing the dielectric ceramic, which method comprises the steps of providing a barium titanate powder in which the c-axis/a-axis ratio in the perovskite structure is about 1.000 or more and less than about 1.003 and the amount of OH groups in the crystal lattice is about 2.0 wt. % or less; and firing the barium titanate powder.
The amount of OH groups is determined based on the loss at 150xc2x0 C. or more as measured during thermogravimetric analysis of specimens.
The barium titanate powder preferably has a maximum particle size of about 0.3 xcexcm or less and an average particle size of about 0.05-0.15 xcexcm.
Also, each particle of the above-described barium titanate powder preferably comprises a low-crystallinity portion and a high-crystallinity portion, and the diameter of the low-crystallinity portion is preferably about 0.5 times or more the particle size of the powder. As shown in FIG. 1, which is a transmission electron microscopic photograph of barium titanate powder, and FIG. 2, which is an explanatory sketch therefor, the term xe2x80x9clow-crystallinity portionxe2x80x9d 21 used herein refers to a domain containing a number of lattice defects such as a void 22, whereas the term xe2x80x9chigh-crystallinity portionxe2x80x9d 23 used herein refers to a domain containing no such lattice defects.
Also, when the ratio (average grain size of fired dielectric ceramic)/(average particle size of provided barium titanate powder) is represented by R, R preferably falls within the range of about 0.90-1.2.
Grains that constitute the dielectric ceramic of the present invention may have a core-shell structure in which the composition and crystal system differ between the core and the shell or a homogeneous structure having a uniform composition and crystal system.
The term xe2x80x9ccrystal systemxe2x80x9d used herein refers to a crystal system of perovskite crystals, i.e., to a cubic system having a c-axis/a-axis ratio in the perovskite structure of about 1 or to a tetragonal system having a c-axis/a-axis ratio in the perovskite structure of about 1 or more.
In yet another aspect of the present invention, there is provided a laminated ceramic electronic element including a laminate formed of a plurality of ceramic layers and an internal conductor formed along a specific interface between adjacent dielectric ceramic layers.
Specifically, in the present invention, the dielectric ceramic layer included in the laminated ceramic electronic element is constituted by a dielectric ceramic obtained by firing a barium titanate powder having a perovskite structure in which the c-axis/a-axis ratio in the perovskite structure is about 1.000 or more and less than about 1.003 and the amount of OH groups in the crystal lattice is about 2.0 wt. % or less.
In the above-described laminated ceramic electronic element, the internal conductors preferably contain a base metal such as nickel or nickel alloy.
The laminated ceramic electronic element may further include a plurality of external electrodes at different positions on a side face or faces. In this case, the internal conductors are formed such that one end of each of the internal conductors is exposed to the side face so as to be electrically connected to one of the external electrodes. Such a structure is typically applied to laminated ceramic capacitors.
In a still further aspect of the present invention, there is provided a method for producing a laminated ceramic electronic element, which method comprises the steps of providing a barium titanate powder in which the c-axis/a-axis ratio in the perovskite structure is about 1.000 or more and less than about 1.003 and the amount of OH groups in the crystal lattice is about 2.0 wt. % or less; fabricating a laminate in which a plurality of ceramic green sheets containing the barium titanate powder and internal electrodes are laminated so that the internal electrodes are present along specific interfaces of the ceramic green sheets; and firing the barium titanate powder to thereby provide a dielectric ceramic.